1. Field of the Invention
In the art of photogrammetry, aerial photographs are taken to cover areas of the earth, and relief distortion which occurs must be eliminated to provide accurate topographic and planimetric maps. This art of preparing such topographic and planimetric maps is popularly known as photogrammetric stereo map plotting. Photogrammetric principles are also utilized in numerous fields to solve problems of mensuration requiring both absolute and relative determinations of size, shape, depth, height and location of objects in their environment. Archaeology, livestock evaluation, medical examinations are but a few such fields, and the utility of the subject invention is not to be considered limited to map-making usage herein described in detail.
Further, by utilizing the apparatus and techniques of the subject invention, actual relief models may be conveniently constructed and utilized for planning purposes or otherwise.
2. Prior Art
Photogrammetric stereo plotters generally in use today comprise a pair of overhead projectors which are utilized to project stereo images from a pair of diapositives into the space above the mapping surface. More precisely, in standard double-projection direct-viewing plotting systems, the images on the diapositives derived from original aerial negatives are illuminated and projected through lenses onto a platen where the projected image is viewed by reflective light.
The projectors are provided with rotational (about each of three mutually perpendicular axes) and linear translational motions to permit the same relative spatial orientations of said projectors above the mapping surface as the orientation of the aerial camera above a specified datum at the instant the aerial photographs were taken. When the projectors are oriented, the light rays from the common images intersect at a point above the mapping surface and thus by pinpointing the intersection of all common image points, a three-dimensional spatial model is created.
A movable platen, which can be vertically adjusted to a position where the common image points intersect, is utilized above a tracing table which forms the mapping surface. The platen is usually a round metal member about 4" in diameter which has a reference mark located on its center, which appears to float in the stereoscopic model. When the floating mark is brought into contact with any point of the apparent surface of the model as viewed on the platen, the precise position of that image is established and can be pinpointed with distortion-free accuracy on the tracing table below. The platen can be guided freely through the stereo model by the plotter operator while maintaining the floating mark in contact with the surface of the model at the point of observation. Thus, by such a method, maps may be plotted by pencil or other means as the various relief and other map features are observed in the locations designated by the image intersections.
By establishing reference points, the exact height of various terrain features can be determined and contour maps created by utilizing the above described platen with its reference mark which may be a hole in the platen which enables the omission of light from beneath to create the intersection of the images. As the light is followed, the plotter operator, with the use of three-dimensional glasses, effectively has a three-dimensional model before him and can follow the light dot enabling him to plot contours effectively reproducing the three-dimensional image as a two-dimensional image.
In addition to manual plotting, an electronic system such as the system known as ORTHOPHOTO also is utilized for map plotting wherein the need for manual planimetric plotting is eliminated. With this method, a flexible lens is utilized and the image is transferred onto a tape by computers, or onto a sensitized film.
In the case of the manual plotting, the map making process is extremely slow, and the skill and accuracy of the plotter operator are of considerable significance. While the ORTHOPHOTO electronic system eliminates some of the need for plotter operator skills and reduces the time necesary for map plotting, the cost of such a system is prohibitive for many organizations. Maintenance costs add to the burden of the expense for an electronic system.
In manual planimetric plotting, details such as buildings are occasionally inadvertently eliminated, and mistakes such as plotting of shadows sometimes occur.